1. Field of the Invention
The present invention relates generally to radar processing systems, and particularly to Ground Moving Target Indicator (GMTI) radar.
2. Technical Background
The term radar is an acronym that stands for “radio detection and ranging.” A radar system transmits radio frequency (RF) signals in a predetermined direction (i.e., a bearing) with the intention of contacting or illuminating moving or stationary objects, depending on the radar type, (“targets”). When the transmitted radar signal illuminates a target, a return signal is reflected back toward the radar receiver. The return signal is detected if it is stronger than the noise signals that may be present in the receiver. A target's bearing corresponds to the direction of the transmitted radar signal. Because the signal travels at the speed of light, the distance, or “range”, is determined by measuring the time between signal transmission and the reception of the return signal. Radar has proved to be a very useful tool that can detect targets such as spacecraft, aircraft, vehicles, etc., within a predetermined region or search volume and provide the radar with the targets bearing, range, velocity, etc. This information provides military commanders, security personnel, or police with the intelligence they need to properly assess their situational awareness. Moreover, radar systems are now being used in many different applications including civilian air traffic control, search and reconnaissance, weather forecasting and tracking, and automotive traffic control, to name a few. Another radar application that has long garnered a great deal of interest relates to ground moving target indication (GMTI).
GMTI is an important application for “look-down” (i.e., airborne and space based systems) radar systems to sense ground targets using their motion. Because resources are limited, military commanders must use their assets smartly and efficiently. To do this, they require reliable intelligence in order to develop accurate “situation awareness” (SA). SA is about knowing where the enemy is, how big it is, where it is going and how fast it is getting there.
One of the drawbacks with GMTI radar relates to its ability (or inability) to distinguish slow-moving targets from background clutter. Clutter refers to the radar return signals that are reflected by terrain, buildings, trees and other such objects that are not of interest to the decision makers. GMTI radars use the Doppler Effect to distinguish moving targets from stationary ones. (When a target approaches the radar receiver, its velocity component parallel to the line of sight of the radar imparts a positive frequency shift if moving towards the radar, and a negative frequency shift if moving away from the radar. This frequency shift is referred to as Doppler and the relevant velocity component is the Doppler velocity. The use of Doppler radar provides a widely used means for distinguishing a target from stationary background (clutter). The Doppler frequency is calculated by calculating the ratio of twice the radial velocity over the wavelength of the radar signal (i.e., FD=(2)(VR)/λ). (This expression assumes a monostatic radar wherein the transmitter and receiver are collocated. In bistatic radar, the expression is modified to account for the differing velocity vector orientation with respect to the transmitter and receiver.) When a radar platform is moving (e.g., it is mounted on an aircraft), however, clutter returns at different angles will appear to move at different velocities and thus impart a spread of Doppler frequencies that can mask a moving target. By filtering both in angle and in Doppler, the radar processor can distinguish between clutter and target unless the target is moving too slowly. In this case, the competing clutter will arise from nearly the same angle and velocity as that of the target. This gives rise to the notion of “minimum detectable velocity (MDV).” Briefly stated, if the target is below the radar's MDV it will not be detected; on the other hand, if a target's Doppler velocity is above the radar's MDV, the GMTI radar can detect the target.
A limiting factor of GMTI radar arises from the fact that the MDV is primarily limited by the electrical size of the radar antenna aperture; sharper angle filtering requires a larger antenna aperture. The MDV is inversely related to the size of the radar antenna aperture. Thus, the MDV is reduced by increasing the size of the radar antenna aperture. However, since the GMTI radar (and its antenna) is part of the aircraft's payload, the size of the antenna aperture is limited by the size of the aircraft platform itself. What is needed therefore is a way to increase the size of the radar antenna aperture without the physical constraints outlined above.
In one approach that was considered, additional antennas were mounted on platforms maintained at a fixed separation. The antennas were widely separated spatially to increase the electrical size of the overall radar antenna aperture. Since the radar processor knew the precise position of each platform's antenna phase center, it also knew a priori what the phase offsets were between antennas. In other words, because the platforms were at a fixed separation, the computational burden placed on the processor was significantly reduced, making the system feasible. One obvious drawback to this approach relates to the fact that the system is rigid during usage.
Thus, a major drawback to mobile multiplatform GMTI radar relates to the fact that the relative positions of the platform antenna phase centers must be tracked to a small fraction of a wavelength. Conventional proposals to solve this ambiguity problem, and other problems associated with mobile distributed array multiplatform radar, make the assumption that the array phase centers can be precision tracked to fractions of a wavelength by some means, or that the array can be cohered by focusing on strong scatterers, transponders, and so forth. Such tracking accuracies are very difficult, if not impossible, to achieve with moving platforms at radar frequencies.
In one approach to mobile multiplatform GMTI radar, a multiplatform airborne experiment was performed. Specifically, the multiplatform radar operated as a mainlobe canceler, with one radar functioning as a main radar channel and the other(s) operating as auxiliary channels. Two drawbacks for this method were discovered. The first drawback relates to the fact that one of the radars alone (in the multiplatform system) must have adequate signal-to-noise ratio (SNR) for detection. Stated differently, the target scattered signals originating from the radar transmitters and received by the radars cannot be made to add coherently. Thus, the coherent gain of the multiplatform radar is lost. The second drawback relates to the lack of monopulse accuracy. Typically, the target angle is estimated accurately by combining two or more phase centers on one platform radar in a monopulse combiner. The combiner is calibrated to relate angle of arrival to the monopulse voltage ratio. The problem with the multiplatform radar is that the calibration curve does not apply if the auxiliary platform radar signals are combined with those of the main radar to cancel clutter. The clutter must be canceled in both monopulse channels.
What is needed, therefore, is a GMTI radar that can detect ground moving targets with very small minimum detectable velocities by combining signals in a clutter suppressing adaptive processing filter without requiring that the relative positions of the antenna phase centers be accurately tracked. What is further needed is a means for providing adequate signal-to-noise ratio (SNR) for detection. A method for canceling clutter in both monopulse channels is also needed.